The preparation of polyisocyanates containing isocyanurate groups has been known for a long time and is described in a large number of publications (e.g. Houben-Weyl, Methoden der organischen Chemie Volume 8, p. 136 ff., Georg Thieme verlag Stuttgart 1952; H. Wagner, H. F. Sarx, Lackkunstharze 5th Edition, page 153 ff., Carl Hanser Verlag Munich 1971; DE-A 4 428 107, U.S. Pat. No. No. 2,993,870; DE-A 1 201 992; DE-A 2 452 532; J. prakt. Chem. 336, pp. 185-200, 1994). Both trimers based on aliphatic and trimers based on aromatic diisocyanates are employed universally as paint base materials and also as polyurethane elastomers and polyurethane foams.
From an occupational hygiene standpoint low-monomer-content trimer product grades are preferred. These products are prepared either by distillative separation of the excess monomer after the trimerization reaction or by stirring the trimerization reaction toward high conversions, until the monomer has very largely undergone conversion to higher-oligomer isocyanurates. The latter method is preferentially successful when the diisocyanates used carry two isocyanate groups which differ in their reactivity, as in the case of 2,4-toluene diisocyanate. Solvent-containing products of this kind can be prepared in this way with a monomeric TDI (sum of the isomeric toluene diisocyanates) content of <0.5% (e.g. ®Desmodur IL, commercial product of Bayer AG, 50% in butyl acetate, NCO content: 8.0%).
As a result of the tightening in the labelling of TDI-based products, interest in substantially monomer-free grades, i.e. products having TDI contents of <0.1%, has come sharply to the fore. In order to achieve this aim, the starting base materials of TDI-based coating systems ought to contain substantially <0.5% by weight TDI, preferably <0.1% by weight.
In principle, this objective can be achieved simply by continuing trimerization to even higher conversions and thus even higher molecular weights. On the one hand, however, this is at the expense of a low viscosity, and on the other hand this approach leads to products having fewer and fewer reactive isocyanate groups. Moreover, this measure is increasingly influencing compatibility in the case of blending with other paint base materials.
There has, however, been no lack of attempts to reduce the monomer content in other ways.
The use of non-solvents for trimers in order to shift the equilibrium is described in, for example, JP-A 56 059 828. Distillation processes for “thin-filmed TDI trimers” (e.g. DE-A 3 420 923, DE-A 19 618 230, DE-A 712 840) and processes which employ a pretreatment or aftertreatment stage, such as subsequent urethanization, for example, are known (e.g. DE-A 3 928 503, U.S. Pat. No. 3,384,624, DE-A 2 414 413, DE-A 19 523 657, DE-A 19 523 657; JP-A2 000 273 142). Even particularly gentle processes for selective trimerization at low temperatures are discussed (JP-A 63 260 915). All of the methods described, however, lead to very complex, often multistage, industrial processes, are associated with very long reaction times and hence a poor space/time yield, and/or cannot be transferred from the laboratory stage to the industrial scale.